But the retina is curved too, so I am not sure there is even a way to convince flat-earthers?

But not on the ground.

There are buildings that go higher than 1,800 feet^. There are many kinds of vehicles that could complete this mission, with some available for charter at very affordable rates. This is all part of why this project is fantastic. It's nonsensical in an exciting way. I really hope he launches (and doesn't die). Hell, I hope he proves the earth is flat.

^ He does plan to go higher in future missions, just to be fair. This is not passive flat earthism. He's starting a flat earth space program, not being trapped on a disc forever.